Polymer 1D photonic crystals

ABSTRACT

A multilayer nonlinear dielectric optical structure is formed by coextruding at least two polymeric materials, components (a) and (b), using a multiplying element; the structure contains a plurality of alternating layers (A) and (B) represented by formula (AB) x , where x=2 n , and n is the number of multiplying elements; at least one of the components (a) and (b) exhibits nonlinear optical response. These structures perform a variety of nonlinear optical functions including all-optical switching and passive optical limiting.

[0001] This application is a continuation of application Ser. No.09/794,492, filed Feb. 28, 2001; application Ser. No. 09/794,492 claimspriority to U.S. Provisional patent application Serial No. 60/195,695,filed Apr. 7, 2000, both of which are incorporated herein by referencein their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to the materials and a method forfabricating multilayer nonlinear dielectric optical structures frompolymeric materials. In particular, the present invention relates tomultilayer materials characterized by a modulation in the nonlinearrefractive index in the direction normal to the surface of the layers.

[0004] 2. Discussion of the Related Art

[0005] The propagation of electromagnetic waves through periodicstructures consisting of layers of materials with an intensity-dependentdielectric constant has been studied both theoretically andexperimentally. A historical review was reported by Brown et al ( T. G.Brown and B. J. Eggleton, Optics Express 3, 385 (1998)). Thetransmission and reflection properties of such structures are stronglymodulated by the intensity of the incident light. The optical responseof such structures can include optical switching, optical limiting,optical bistability and some remarkable pulse propagation effectsincluding transverse pattern formation.

[0006] Some optical effects have been shown experimentally using, forexample, colloidal arrays (C. Herbert and M. Malcuit.; Opt. Lett. 17,1037 (1992)), semiconductor multilayers, silicon-on-insulator waveguides(N. D. Sankey, D. F. Prelewitz and T. G. Brown; Appl. Phys Lett. 60,1427 (1992)) and fiber gratings (S. Larochelle, Y. Hibino, V. Mizrahiand G. Stegeman; Electron. Lett. 26, 1459 (1990)). An organic nonlineardielectric stack was reported by Norwood et al. in 1992 using a siliconnaphthalocyaine-poly(methyl methacrylate) structure made by spin coatingsequential layers (R. A. Norwood et al.; Opt. Lett. 17, 577 (1992)).However, only a few layers are possible with a spin coating technique.Norwood et al in the above publication reports only 23 layers.

[0007] The preparation of layered structures of polymeric materials bycoextrusion has been used to prepare materials with combinations ofphysical properties. Such materials having three to seven layers arecommercially available. More recently, the development of layermultiplying dies has allowed the preparation of multilayer polymericmaterials with hundreds or even thousands of layers (E. Baer, J. Kerns,and A. Hiltner; “Processing and Properties of Polymer MicrolayeredSystems: NATO-ASI on Structure Development During Polymer Processing,Guimaraes, Portugal, May 17-28, (1999)). The total thickness of thestructured material is controlled by the feed ratio. With thousands oflayers, individual layer thickness down to 30 nanometers or less can beachieved. Recent processing improvements and improvements in themultiplying elements allow layer thickness to be constant within a fewpercent.

[0008] Optical properties of nonlinear dyes have led to the developmentof materials that have a large nonlinear absorption and/or a largenonlinear refraction coefficients (J. S. Shirk, R. G. S. Pong, F. J.Bartoli, A. W. Snow; Appl. Phys. Lett., 63, 1880 (1993)). In thesematerials, the nonlinear response includes contributions from excitedstate absorption, excited state nonlinear refraction and thermalrefraction. Some of these materials are soluble in polymeric materialsthat are suitable for the microlayer extrusion process.

[0009] Accordingly, there is a need to develop multilayer structuresthat exhibit improved nonlinear optical response.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method and materials forfabrication of a multilayer structure comprising a plurality of at leasttwo alternating layers (A) and (B) represented by formula (AB)_(x),where x=2^(n), and n is in the range of from 2 to 15. Layer (A) iscomprised of component (a) and layer (B) is comprised of component (b),where at least one of components (a) and (b) exhibits nonlinear opticalresponse.

BRIEF DESCRIPTION OF THE DRAWING

[0011]FIG. 1 is a schematic representation of an arrangement forcoextrusion of two components (a) and (b) to form a multilayer structureof alternating layers (A) and (B), in accordance with the invention.

[0012]FIG. 2 shows a series of multiplying elements.

[0013]FIG. 3 is a schematic representation of a flow diagram for a threelayer system (A), (B) and (T), where (T) is a tie layer.

[0014]FIG. 4 shows the nanosecond transmission apparatus. The sample ismounted on a translation stage near the focus. A1 and A2 are theentrance and exit apertures L1, L2 and L3 are lenses used to focus thelight on the sample film, recollimate it and weakly focus it on thedetector (Det). Ref is the reference detector.

[0015]FIG. 5 shows the reflection spectra of 5 mil and 14 milnanolayered SAN25/SAN20 films

[0016]FIG. 6 shows the Atomic Force Microscopy (AFM) phase image ofnanolayered SAN25+ nonlinear dye/SAN20 film

[0017]FIG. 7 shows the reflection spectra of 4096 layer SAN20/SAN25+nonlinear dye films with different layer thickness. In the upperspectrum the average layer thickness is 87 nm, in the lower it is 31 nm.

[0018]FIG. 8 shows the closed aperture Z-scan of multilayer nonlinearfilm of polymeric material at 532 nm. The film shows a negativenonlinear refractive index change with incident fluence.

[0019]FIG. 9 shows Transmission vs. Energy for a nigrosine dyednanolayered film at 532 nm. The film had 4096 layers with an averagethickness each of 87 nm.

[0020]FIG. 10 shows Open Aperture Z-scan for a leadtetrakis(cumylphenoxy)phthalocyanine (PbPc(PC)₄) dyed multilayer film at532 nm.

[0021]FIG. 11 shows reflection spectra of 4096 layerpolycarbonate/polycarbonate+nigrosine dye films with different layerthickness.

[0022]FIG. 12 shows reflectivity at 573 nm as a function of distancefrom focus for a 14 mil, 4096 layer,polycarbonate/polycarbonate+nigrosine dye film (lower curve) compared tothat for a film where the layers are identical (upper curve).

[0023]FIG. 13 is a schematic representation of a multilayer opticalswitch in accordance with the present invention. Initially, therefractive indices of the different layers are close to being matched,and the sample transmission is high (A). When the control beam ispresent (or the signal beam becomes sufficiently intense), therefractive index of the layers is not matched. The resulting indexmodulation makes the material an effective dielectric mirror (B). Thereflectivity increases and the transmission falls.

[0024]FIG. 14 shows the reflectivity as a function of time for a 4096layer sample, in accordance with the present invention, where thealternate layers consisted of undoped polycarbonate and the same polymercontaining 0.2% (wt/wt) nigrosine. The sample was mounted between glassplates. The film had an average layer thickness of 87 nanometers (nm).

[0025]FIG. 15 shows the reflectivity as a function of time for a freestanding 4096 layer sample, in accordance with the present invention,similar to that in FIG. 14. The pump beam was a 1.2 picosecond (ps)pulse at time equal 0 on the graph. The pump energy was about threetimes that in FIG. 14.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The multilayer structure of the present invention is a nonlineardielectric optical material that has a substantial modulation in thenonlinear dielectric constant across the sample. A material with amodulation in the linear dielectric constant is commonly called a“photonic crystal” (J. D. Joannopoulos, R. D. Meade, and J. N. Winn;“Photonic Crystals” (Princeton Univ. Press, (1995)) when the period ofthe modulation is on the order of a wavelength of light. Thus thematerials described here, which possess a modulation in the nonlineardielectric constant across the sample, can be considered as ID nonlinearphotonic crystals. These materials exhibit an intensity dependenttransmission and reflection. They act as nonlinear mirrors and perform avariety of functions including optical switching and optical limiting.

[0027] Accordingly, it is an object of the present invention to providea multilayer structure as nonlinear multilayer 1-D photonic crystal anda method for the fabrication of same.

[0028] In one embodiment of the present invention the multilayerstructure is made of two alternating layers (ABABA . . . ) of twopolymeric materials referred to as component “(a)” and component “(b)”,respectively, throughout the description.

[0029] Materials

[0030] One of ordinary skill in the art will readily appreciate that awide variety of materials can be used to form the multilayer structureof the present invention. The components comprising the different layersof the multilayer structure are polymeric materials chosen to have adifference in the index of refraction of the layers preferably on theorder of from 0 to 10%, including any increments within that range, mostpreferably on the order of 0 to 2%. The degree of index matching ischosen to provide the desired initial level of reflectivity and tomaximize the change in reflectivity with nonlinear index. By way ofexample, the preferred index matching for polycarbonate (PC) with arefractive index n₀=1.583, is for the alternate layers to be matched towithin 0.004 with the nonlinear layer having the smaller index. Thecomponent comprising the alternating layers are preferably a polymericmaterial, a polymeric composite material, an oligomeric material, and/ora polymeric material containing a nonlinear dye. The content of thenonlinear dye in the polymeric material is preferably in the order of0.1 to 5 wt %. Further, it is preferred that a good inter-layer adhesionbetween co-extruded layers is exhibited in the multilayer structure toreduce the possibility of delamination during end use.

[0031] The term “polymeric material” as used in the present applicationdenotes a material having a weight average molecular weight (Mw) of atleast 5,000. Preferably the polymeric material is an organic polymericmaterial. The term “polymeric composite material” as used in the presentapplication denotes a combination of a polymeric material with at leastone more material dispersed therein; the additional material can beanother polymeric material or an inorganic material; examples of suchinorganic materials include inorganic fillers, such as glass, titaniumdioxide and talc; further, the inorganic material may be the form ofparticles, rods, fibers, plates etc. It is preferred that the compositematerial is substantially optically transparent. Accordingly, it ispreferred that the dispersed material is miscible with the polymericmaterials, has a refractive index substantially the same with thepolymeric material or is finely dispersed to avoid light scattering.Such composite materials are a convenient and useful way to control thelinear part of the refractive index in the individual layers. The term“oligomeric material” as used in the present application denotesmaterial with a degree of polymerization (DP) between 10 and 1000. Theterm “nonlinear dye” as used in the present application denotes amaterial whose dielectric constant and hence the refractive index varieswith the incident light intensity and/or the incident light fluence. Theintensity or fluence dependence can be in either or both the real orimaginary part (absorptive part) of the refractive index. The term“nonlinear optical response” of a material, as used in the presentapplication, denotes that the real and/or the imaginary part of therefractive index of the material is a function of the intensity and/orfluence of the incident light. Specifically, either the absorbanceand/or the refractive index varies with the fluence (intensity). For anoptical limiter, a dye with a nonlinear optical response that includesan absorbance that can increase with fluence (intensity) is particularlyuseful. Further, the multilayer structure of the present invention is a1D photonic crystal in the sense that a photonic crystal is a materialhaving a spatially periodic modulation of the refractive index. Further,the term “nanolayer” is used to denote a layer with thickness innanoscale.

[0032] Suitable polymeric materials in accordance with the presentinvention include but are not limited to, polyethylene naphthalate andisomers thereof such as 2,6-, 1,4-, 1,5-, 2,7-, and 2,3- polyethylenenaphthalate; polyalkylene terephthalates such as polyethyleneterephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate; polyimides such aspolyacrylic imides; polyetherimides; styrenic polymers such as atactic,isotactic and syndiotactic polystyrene, α-methyl-polystyrene,para-methyl-polystyrene; polycarbonates such asbisphenol-A-polycarbonate (PC); poly(meth)acrylates such aspoly(isobutyl methacrylate), poly(propyl methacrylate), poly(ethylmethacrylate), poly(methyl methacrylate), poly(butyl acrylate) andpoly(methyl acrylate) (the term “(meth)acrylate” is used herein todenote acrylate or methacrylate); cellulose derivatives such as ethylcellulose, cellulose acetate, cellulose propionate, cellulose acetatebutyrate, and cellulose nitrate; polyalkylene polymers such aspolyethylene, polypropylene, polybutylene, polyisobutylene, andpoly(4-methyl)pentene; fluorinated polymers such as perfluoroalkoxyresins, polytetrafluoroethylene, fluorinated ethylene-propylenecopolymers, polyvinylidene fluoride, and polychlorotrifluoroethylene;chlorinated polymers such as polydichlorostyrene, polyvinylidenechloride and polyvinylchloride; polysulfones; polyethersulfones;polyacrylonitrile; polyamides; polyvinylacetate; polyether-amides. Alsosuitable are copolymers such as styrene-acrylornitrile copolymer (SAN),containing between 10 and 50 wt %, preferably between 20 and 40 wt %,acrylonitrile, styrene-ethylene copolymer; andpoly(ethylene-1,4-cyclohexylenedimethylene terephthalate) (PETG). Inaddition, each individual layer may include blends of two or more of theabove-described polymers or copolymers. Preferred polymeric materialsinclude a styrene-acrylonitrile copolymer and a polycarbonate.

[0033] Nonlinear dyes can be used to confer nonlinear absorption,nonlinear refraction or both on the polymeric materials and polymercomposites used in the multilayer samples. Nonlinear dyes can also beused to enhance the nonlinear optical response of polymers with aninherent nonlinear optical response. The nonlinear absorption propertyis obtained by using nonlinear dyes that exhibit reverse saturableabsorption, sequential two-photon absorption, or two-photon absorptionwith sequential two-photon absorption or reverse saturable absorptionbeing preferred. Nonlinear dyes exhibiting this property includephthalocyanines, naphthalocyanines, porphyrins, organometallic clustercompounds, and fullerenes. Some preferred materials are leadtetrakis(cumylphenoxy)phthalocyanine (PbPc(CP)₄), leadtetrakis(polydimethylsiloxane) phthalocyanine, tetra-tert.-butyl(para-trifluoromethyl phenyl) indium(III) phthalocyanine andbis(trihexylsiloxy)silicon naphthalocyanine. The nonlinear refractionproperty is obtained by using nonlinear dyes or a polymeric materialthat exhibits an intensity dependent refractive index. The intensitydependent refractive index can be obtained by either thermal expansionin a material with a substantial dn/dT (where n is the refractive indexand T is the temperature) by optical pumping to an excited state wherethe excited state has a different refractive index from the groundstate, by a resonant x⁽³⁾ (where x⁽³⁾ is the third order susceptibility)where the resonance is provided by two-photon absorption, for example,or by a nonresonant x⁽³⁾.

[0034] The preferred mechanism is thermal or optical pumping. Nonlineardyes exhibiting these properties include nigrosine, cyanines,phthalocyanines, naphthalocyanines, porphyrins, organometallic clustercompounds, carbon blacks and fullerenes. The host polymeric materialcontributes to the thermal mechanism, so polymeric materials with largedρ/dT ( where ρ is the density) are preferred. Preferred host polymericmaterials include polycarbonates, polystyrenes, poly(methyl)methacrylateand polysiloxanes. The thermal nonlinearity is enhanced by theperipheral substituent on the nonlinear dye. For a thermal nonlinearity,nigrosine, and copper tetrakis(cumylphenoxy)phthalocyanine arepreferred; for an optical pumping nonlinearity leadtetrakis(cumylphenoxy)phthalocyanine (PbPc(CP)₄ ), leadtetrakis(polydimethylsiloxane) phthalocyanine or the equivalentbis(trihexylsiloxy)-silicon naphthalocyanines are preferred nonlineardyes.

[0035] Suitable examples of polymeric materials with inherent nonlinearresponse include, but are not limited to, polyacetylene, polythiophene,poly(diacetylene), poly(p-phenylene vinylene), poly(thienylene vinylene)and poly(p-phenylene benzobisthiazole).

[0036] Fabrication

[0037] The 1-D photonic crystal is then fabricated using these materialsin a multilayer extrusion technique. The method preferably yields aflexible large film or sheet of multilayer nonlinear dielectric opticalstructure. The thickness of the individual layers is on the order offrom 5 nanometers to 10 micrometers, preferably from 10 nanometers to1000 nanometers, more preferably from 30 nanometers to 200 nanometers.They are engineered to provide reflectivity within a wavelength regionbetween about 200 micrometers and the near UV (350 nanometers). In otherwords, there should be a transmission window somewhere between 200micrometers and 350 nanometers. The term “about” is used in the presentapplication to denote a deviation from the stated value. Preferably, thepolymeric materials used in the alternating layers are transparent inthe above region. The layer thickness can be varied or chirped toprovide variable reflectivity over a broad band of wavelengths andacceptance angles. Preferably, the layers have substantially uniformlayer thickness, where “substantially” is used to denote a deviationwithin 20%.

[0038] For simplicity of discussion, the behavior of a two componentsystem is described. In this embodiment of the present invention themultilayer structure is made of two alternating layers (ABABA . . . ) oftwo polymeric materials referred to as component “(a)” and component“(b)”, respectively, throughout the description. The components (a) and(b), may be the same or different and form a multilayer structurerepresented by formula (AB)_(x), where x=(2)^(n), and n is the number ofmultiplier elements. At least one of components (a) and (b) exhibitsnonlinear optical response. It should be understood that the multilayerstructure of the invention may include additional types of layers. Thecomponents of the various alternating layers may be the same ordifferent as long as at least one component exhibits nonlinear opticalresponse. For instance, a three component structure of alternatinglayers (ABCABCA . . . ) of components (a), (b) and (c) is represented by(ABC)_(x), where x is as defined above.

[0039] In the two-component system described above one of thealternating layers (A) comprises component (a) which is a polymericmaterial with a dissolved nonlinear dye and a second alternating layer(B) comprises component (b) which is a polymeric material alone. It isdesirable to adjust the composition so that the difference between thelinear index of the layer containing the nonlinear dye and the linearindex of the polymeric material alone is between 0 and 10% including anyincrements therein, preferably between 0 and 2%. The multilayerstructure in the above embodiment is represented by formula (AB)_(x),where x=(2)^(n), and n is the number of multiplier elements.

[0040] In the embodiment described above of a two-component multilayerstructure, the 1-D photonic crystal is prepared by microlayercoextrusion of the two polymeric materials. Nanolayers are comprised ofalternating layers of two or more components with individual layerthickness ranging from the microscale to the nanoscale. A typicalmultilayer coextrusion apparatus is illustrated in FIG. 1. The twocomponent (AB) coextrusion system consists of two ¾ inch single screwextruders each connected by a melt pump to a coextrusion feedblock. Thefeedblock for this two component system combines polymeric material (a)and polymeric material (b) in an (AB) layer configuration. The meltpumps control the two melt streams that are combined in the feedblock astwo parallel layers. By adjusting the melt pump speed, the relativelayer thickness, that is, the ratio of A to B, can be varied. From thefeedblock, the melt goes through a series of multiplying elements. Amultiplying element first slices the AB structure vertically, andsubsequently spreads the melt horizontally. The flowing streamsrecombine, doubling the number of layers. An assembly of n multiplierelements produces an extrudate with the layer sequence (AB)_(x) where xis equal to (2)^(n) and n is the number of multiplying elements. It isunderstood by those skilled in the art that the number of extruders usedto fabricate the structure of the invention equals the number ofcomponents. Thus, a three-component multilayer (ABC . . . ), requiresthree extruders.

[0041] The multilayer structure of the present invention preferably hasat least 30 layers, including any number of layers within that range.Preferably, the multilayer structure of the present invention has from50 to 10000 layers. Preferably, the multilayer structure is in the formof film or sheet. By altering the relative flow rates or the number oflayers, while keeping the film or sheet thickness constant, theindividual layer thickness can be controlled. The multilayer structurefilm or sheet has an overall thickness ranging from 10 nanometers to1000 mils, preferably from 0.1 mils to 125 mils and any incrementstherein. Further, the multilayer structures may be formed into a numberof articles. The structures may be formed by coextrusion techniquesinitially into films or sheets which may then be post formed. Such postforming operations may include thermoforming, vacuum forming, orpressure forming. Further, through the use of forming dies, themultilayer structures may be formed into a variety of useful shapesincluding profiles, tubes and the like.

[0042] Structures exhibiting unique optics can be produced withmicrolayer processing technology. The present inventors have found thatlayering improves the nonlinear optical response of a multilayerstructure. In other words, the larger the number of layering the betterthe nonlinear optical response of the multilayer structure. Inaccordance with the present invention, when two materials of differingrefractive indices are extruded into a multilayer structure that haslayer thickness of approximately ¼ the wavelength of visible light, thematerial will be reflective. Increasing the number of layers increasesthe number of interfaces, and thus amplifies the reflective response.

[0043] In another embodiment of the invention, a third polymeric layeris placed in the multilayer structure as a tie layer, barrier layer ortoughening layer. A multilayer structure containing a third componentexhibits improved properties such as mechanical properties. Thus, athree component multilayer structure expands the utility of the twocomponent nanolayer structure. When the third polymer layer is a barrierlayer, it is present as a single layer on one or both exterior majorsurfaces of the structure or as an interior layer. For example, suitablebarrier layer materials such as hydrolyzed ethylene vinyl acetate,copolymers of polyvinylidene chloride, nitrile polymers, and nylons maybe used in or on the multilayer body. Suitable adhesive materials suchas maleic anhydride grafted polyolefins may be used to bond such barrierlayer materials to the multilayer structure. Alternatively, a thirdpolymeric layer may be used as a surface or skin layer on one or bothmajor exterior surfaces. The skin layer may serve as scratch resistant,weatherable protective layer, as sacrificial layer or as decorativelayer. Further, such skin layers may be post applied to the structureafter coextrusion. A typical three component system according to theabove embodiment is illustrated in FIG. 3. A tie layer (T) is insertedbetween layer (A) and layer (B) by using a five layer feedblock. Thefive layer melt is sliced, then spread and recombined to give a layersequence (ATBT)_(x)A, where x is equal to (2)^(n) for an assembly of nmultiplier elements. The additional polymeric layer may be a barrierlayer

[0044] Nonlinear Response

[0045] Thermal changes in the index of refraction are generally largest.These require a nonlinear dye that converts the absorbed energy to heatefficiently and rapidly and a polymeric material host that has a largechange in index or refraction with temperature. Nigrosine is a typicalexample of this class of nonlinear dyes. Another class of materials thatcan be used as the nonlinear dye is a phthalocyanine, a porphyrin and anaphthalocyanine (H. S. Nalwa, and J. S. Shirk, “Nonlinear OpticalProperties of Metallophthalocyanines” in Phihalocyanines: Properties andApplication, C. C. Leznoff and A. B. P. Lever, Eds. ,VCH Publishers, NewYork, 4, 79 (1996)). Nonlinear dyes of this type are appropriate if theyare substituted at the peripheral or axial positions to providesolubility and compatibility with the polymeric material host. PbPc(CP)₄in a styrene/acrylonitrile copolymer host is an example. These nonlineardyes usually possess a nonlinear absorption as well as a nonlinearrefraction, so the absorption increases with the incident fluence. Theincreasing absorption serves to enhance the thermal nonlinear mechanism.It also makes these materials particularly suitable for use as opticallimiter.

[0046] Optical Switches

[0047] The nanolayered materials of the present invention can act asoptical switches. The basic principle for nonlinear reflection using thenanostructured material can be understood from FIG. 13. The opticalswitch consists of alternate layers of a linear polymer and a polymerwhose index of refraction varies with light intensity. Initially, therefractive indices of the different layers are close to being matched,and the sample transmission is high (A). When the control beam ispresent, the refractive index of the layers is not matched. Theresulting index modulation, with a spacing of λ/4, makes the material aneffective dielectric mirror (B). The increased reflectivity will causethe signal beam to be reflected.

[0048] These materials can also be self-acting switches. If the signalbeam becomes sufficiently intense, it will cause the reflectivity toincrease and the transmission to fall. This property is useful foroptical limiting applications.

[0049] The response time of the optical switches, in accordance with thepresent invention, is preferably at least 20 femtosecond, morepreferably on the order of 1 to 1000 picoseconds, and most preferablybetween 10 to 500 picoseconds, including any increments within the aboveranges.

[0050] Optical Limiters

[0051] In another embodiment of the present invention, the multilayerstructure of the can be used as an optical limiter. A material that hasthe multilayer structure of the present invention will provide alimiting effect superior to that found in equivalent homogeneousmaterials. In current optical limiters, nonlinear absorption, nonlinearrefraction and nonlinear scattering all are contributing mechanisms.Nonlinear absorption, where the material absorbance increases with theintensity of the incident light, is a good mechanism to remove lightfrom the transmitted beam. Nonlinear refraction can be used to deflector scatter light out of the transmitted beam, thereby decreasing thedevice transmission. When the nonlinear material has a refractive indexthat depends on the light intensity or fluence, the intensitydistribution across the beam generates a refractive index variation inthe nonlinear material that behaves as a lens with strong aberrations.This lens will deflect some of the light into the wings of thetransmitted beam where an exit aperture blocks it. This serves to limitthe energy in the transmitted beam at high input fluences.

[0052] All these optical limiting mechanisms will be effective when thelimiter material possesses the multilayer structure of the presentinvention. In addition, the multilayer structures will provide anoptical switching mechanism, as described above, that will contribute toand enhance the optical limiting. Further, the multilayer structure willprovide enhanced nonlinear absorption, since the internal reflectionsinduced within the multilayer structure will increase the effective pathlength, and reduce the transmission.

[0053] Following is a comparison of the present invention with thestructure of the Norwood et al. publication (R. A. Norwood et al.; Opt.Lett. 17, 577 (1992)). The Norwood et al structure used 23 layers of acopolymer of silicon naphthalocyanine/methyl methacrylate alternatingwith methyl methacrylate. The layers were constructed via spin coating.The mechanism of the nonlinear response was saturable absorption. Thismechanism leads to a strong absorption at the operating wavelength.Accordingly:

[0054] 1. The highly absorbing nonlinear layer limits the number oflayers that can be used. This makes the structure much more susceptibleto optical damage. Damage was reported at 15-20 MW/cm² for the Norwoodstructure. In contrast, the multilayer polymer of the present inventionreflectors can sustain greater than 500 MW/cm².

[0055] 2. The limited number of layers possible means the Norwood et aldevice can have only a narrow band reflectivity. The small number oflayers that are possible means that this device cannot achieve a largereflectivity over more than about 20-30 nm. Multilayer samples canexhibit a reflectivity greater than 500 nm.

[0056] 3. The Norwood device gives an increase in transmission and adecrease in reflection with increasing intensity. This response is notuseful as a transmission optical limiter. Further, the narrow bandreflectivity and the strong absorption at the reflecting wavelength arenot useful in a limiter. Multilayer samples show an increase inreflectivity and a decrease in transmission with increasing incidentintensity at wavelengths where the low intensity transmission can begreater than 80%.

[0057] 4. The Norwood et al structure as an optical switch has largeabsorption losses and the peak of only 20% reflectivity at the operatingwavelength. By contrast, the multilayer structure of the presentinvention, operating as optical switches, have greater than 95%reflectivity with absorption losses of less than 1%. The latter makes amore efficient optical switch that can be operated at much higher rates.

[0058] 5. The extrusion technique is capable of making much largerdimensioned multilayer structures of a much wider variety of materials.It also allows the easy fabrication of many more layers than the spincoating technique. Extrusion also permits faster and more economicalfabrication than stepwise spin coating. Since spin coating requires thatthe alternate layers be relatively insoluble, it is not feasible toprepare multilayer structures with polymeric materials containing asoluble dye.

[0059] Having generally described this invention, a furtherunderstanding can be obtained by reference to certain specific exampleswhich are provided herein for purposes of illustration only and are notintended to be limiting unless otherwise specified.

EXAMPLES

[0060] Non-linear dielectric optical structures were made by coextruding4096 nanolayered sheets of alternating dyed and undyedpoly(styrene-co-acrylonitrile). Base refractive indices were tailored bychoosing poly(styrene-acrylonitrile) copolymer (SAN) materials withdiffering acrylonitrile content. Layer thickness in the range of 100 nmwere processsed fabricating 1-D photonic crystals.

[0061] System:2 component.

[0062] Number Of Layers: 4096 Composition Materials 50/50 0/100 1 SAN20/SAN 25 + 5,14 mils — 0.2 wt % Nigrosine Dye 2 SAN 20/SAN 25 + 14 mils— 1 wt % Lead Dye¹ 3 PC/PC + 5,14,20 mils 20 mils 0.2 wt % Nigrosine Dye4 PC/PC + 5,14 mils — 1 wt % Lead Dye¹

[0063] Details of the Mixing Procedure(Polymer-Dye)

[0064] A counter rotating intermeshing twin screw extruder was used formixing the polymeric material and the nonlinear dye. Mixing TemperatureMixing Speed Materials (° C.) (rpm) 1 SAN + 0.2 wt % Nigrosine Dye 23025 2 SAN + 1 wt % Lead Dye¹ 220 25 3 PC + 0.2 wt % Nigrosine Dye 257 214 PC + wt % Lead Dye¹ 257 21

[0065] Details of Multilayer Coextrusion

[0066] Extruders

[0067] Make—KILLION Extruders Inc.

[0068] Size—¾″

[0069] L/D ratio—24:1

[0070] Melt Pumps

[0071] ZENITH Pumps Inc.

[0072] Capacity—1.2 cc/hr

[0073] Multipliers

[0074] Long multipliers were used.

[0075] # of multipliers—11

[0076] Die

[0077] A 3″ die was used.

[0078] Processing Conditions

[0079] 1. SAN 20/SAN25+0.2wt % Nigrosine Dye 1. SAN 20/SAN25 + 0.2 wt %Nigrosine Dye Temperature (° C.) Barrel Zone #1 200 Barrel Zone #2 220Barrel Zone #3 230 Clamp 230 Adapter 230 Pump 230 Multipliers 210 ExitDie 230 Extruder 1 Extruder 2 Screw Speed (rpm) 35 24 Pump Speed (rpm)20 20 2. SAN 20/SAN 25 + 1 wt % Lead Dye¹ Temperature (° C.) Barrel Zone#1 230 Barrel Zone #2 250 Barrel Zone #3 265 Clamp 265 Adapter 265 Pump265 Multipliers 220 Exit Die 240 Extruder 1 Extruder 2 Screw Speed (rpm)25 31 Pump Speed (rpm) 20 20

[0080] 3. PC/PC + 0.2 wt % Nigrosine Dye Temperature (° C.) Barrel Zone#1 220 Barrel Zone #2 250 Barrel Zone #3 270 Clamp 270 Adapter 270 Pump270 Multipliers 210 Exit Die 220 Extruder 1 Extruder 2 Screw Speed (rpm)27 32 Pump Speed (rpm) 17 17 4. PC/PC + 1 wt % Lead Dye¹ Temperature (°C.) Barrel Zone #1 208 Barrel Zone #2 242 Barrel Zone #3 253 Clamp 253Adapter 253 Pump 253 Multipliers 190 Exit Die 200 Extruder 1 Extruder 2Screw Speed (rpm) 3.5 3.8 Pump Speed (rpm) 7.0 7.0

[0081] A description of the linear and nonlinear optical properties ofsome nanolayer polymeric structures fabricated by the extrusion methodfollows. Further, it was shown that the multilayer films exhibit abroadband reflectivity in the visible with relatively small indexdifferences between the layers. It was then shown that the nonlinearnanolayered materials of the present invention exhibited a transmissionand reflectivity that varied with the intensity or fluence of anincident beam. The layered film behaves as a nonlinear ID photoniccrystal.

[0082] Experimental Apparatus

[0083] The samples of the ID photonic materials and control samples werecut directly from the extruded material. Some samples were mountedbetween microscope slides using an optical cement. Mounting reducedscattering from the surface of the extruded polymeric materials.Absorption spectra of each of the samples were recorded on aPERKIN-ELMER LAMBDA 9 spectrophotometer. Absorption spectra of smallregions of the samples were recorded using an OCEAN OPTICS fiber opticspectrometer. Reflection spectra were recorded using the fiber opticsspectrometer with a reflectance probe.

[0084] The nanosecond transmission and reflection experiments wereperformed using the apparatus shown in FIG. 4. The samples were mountedon a translation stage near the focus of a laser. The laser source forthe nanosecond experiments was an optical parametric oscillator (OPO)pumped by the third harmonic of a Nd/YAG laser. The pulse width was 2.5nanoseconds. Some of the nanosecond experiments at 532 nm used a doubledNd/YAG laser with an 8 nanosecond pulse width. In both cases, the inputbeam was spatially filtered to give a spatial profile that typicallyshowed a greater than 96% correlation with a gaussian profile. The laserintensity was controlled by several wave plate/polarizer combinations inseries.

[0085] The experiments at low energies were conducted at a 10 Hzrepetition rate. For incident fluences above 10 mJ/cm², the repetitionrate was reduced to 0.5 Hz in order to reduce any effect due topersistent heating.

[0086] Two different sets of focusing optics were used. In the first,the input beam was focused using approximately f/45 optics. The focalspot size, f/number, and the beam quality were determined from knifeedge scans at several positions along the beam path near the focus. Themeasured M² increased from 1.1 at 530 nm to 1.3 at 610 nm. A diffractionlimited gaussian beam would have M²=1. The measured focal spot size waswithin about 10% of that expected for a gaussian beam between 530 nm and610 nm. The second set of optics was f/S focusing optics. The inputlaser beam was expanded so that only the first aperture transmitted thecentral 10% of the beam. This gives the beam an approximately flat topintensity profile. The lens L1 was a corrected multiplet, which focusedthe light to a spot with a measured beam radius of 2.5±0.1 μm at 532 nm.This was close to the expected radius for a diffraction-limited spotsize, which is 2.2 μm. With the OPO as a source, the beam radius wasmeasured to be 2.6±0.1 μm at 550 nm.

[0087] In both cases, the light passing through the sample wasrecollimated by L2 and focused by lens L3 onto the detector. With noexit aperture (A2), lens L2 in the second optics set providedapproximately f/0.8 collection optics. The film was mounted on atranslation stage and translated through the focus for the Z-scanexperiments. For nonlinear transmission measurements, the position wasadjusted about the focus to give the smallest transmitted energy forincident energies near the limiting threshold.

[0088] In the reflection experiments, the sample was rotated so thatangle of incidence was about 10°. A small pickoff mirror was used todirect reflected light to a third detector next to the referencedetector.

[0089] Optical switching measurements were made by measuring thereflectivity of a multilayer sample as a function of time after aseparate pump beam was incident on the sample. The picosecond (ps), timeresolved, experiments were performed using a dye laser that issynchronously pumped by the second harmonic of a CW mode-locked Nd: YAGlaser. The dye laser output is amplified in a three-stage dye amplifierpumped using the second harmonic of a regenerative Nd: YAG amplifierseeded with the fundamental of the mode-locked Nd: YAG laser. The 10 Hzoutput of the laser system provides 1.2 ps FWHM pulses with energies upto 1 mJ. It was tuned near 590 nm for these experiments. The beam issplit into two or three beams that can be used as pump and probe beams.The intensity of the beams is varied using waveplate/polarizercombinations. The timing between the arrival of the beams at the sampleis achieved by variable optical delay lines. The apparatus is modifiedfrom a degenerate four-wave nixing apparatus that has been described indetail in Shirk et al (Shirk, J. S.; Lindle, J. R.; Bartoli, F. J.;Boyle, M. E., J. Phys. Chem., 96, 5847 (1992)) and Flom et al. (Flom, S.R.; Pong, R. G. S.; Bartoli, F. J.; Kafafi, Z. H. Phys. Rev. B, 46,15598 (1992)) both of which are incorporated herein by reference.

[0090] The reflectivity measurements were made by overlapping a variableintensity pump beam with a probe beam at the sample. The probe beam wasnormal to the polymer surface. The pump beam was incident at 13° fromnormal to the surface of the film. The reflectivity of the film wasmeasured with a detector at the specular reflection angle Timeresolution is obtained by varying the optical path of the pump beam.Additional experiments were performed with the pump beam normal to thesurface of the multilayer sample and the probe beam at 13° from normal.

[0091] Results

[0092] Polycarbonate/poly(methyl methacrylate) ID photonic crystals

[0093] Nanolayered films with alternating polycarbonate (PC) layers andpoly(methyl methacrylate) (PMMA) layers with equal thickness wereprepared. These had from 32 to 4096 layers and the individual layerthickness varied from 12 nm to several microns. These film samples wereup to 12 inches wide and several feet long. The refractive index of PCis about 1.58 and that of PMMA is about 1.49. This index contrast islarge enough so that films with a layer thickness between 65 nm and 115nm exhibit a strong visible reflection. Qualitatively, those films wherethe band gap fell in visible showed a broad, strong, reflectivity in thevisible that was obvious on visual inspection. These films had a silveryappearance. Those films where the layers were much thinner or thickerappeared transparent and showed only the Fresnel reflectivity of a fewpercent.

[0094] Styrene-Acrylonitrile Copolymer 1D Photonic Crystals

[0095] Several nonlinear nanolayered polymeric films were fabricated.The first were made of SAN25, a styrene-acrylonitrile copolymer with 25%acrylonitrile. A nonlinear response was introduced into the alternatelayers of SAN25 by dissolving a nonlinear dye in the polymeric material.The nonlinear dyes used were PbPc(CP)₄ and nigrosine. The nigrosine hasa broad absorption in the visible and an excited state that relaxes onthe sub-picosecond time scale with the conversion of the absorbed energyinto heat. The resulting rise in temperature caused a thermal change inthe refractive index of the host polymeric material. The nigrosine dyedSAN is an example of a thermal refractive nonlinear material. Multilayerfilms consisting of layers of SAN25 doped with a nonlinear dyealternating with an undyed SAN20 polymer (styrene-acrylonitrile with 20%acrylonitrile) layer were also fabricated. The refractive index of theSAN20 was about 0.0045 larger than that of SAN25. Dissolving nigrosinedye in the SAN25 polymer raised the refractive index by approximately0.003. Thus using SAN20 in the linear layers gave a better indexmatching with the dyed layer of the film.

[0096] Linear Optical Properties of the SAN Films

[0097] One of the control samples had alternate layers of SAN20 andSAN25 with no dye in either layer. The index difference between thelayers was about 0.003. This nanolayered film illustrates the broadbandreflectivity that can be achieved with small index differences. FIG. 5shows the reflection spectrum of two of these materials, The one labeled14 mil had 4096 layers with an average thickness of 87 nm and thatlabeled 5 mil had 4096 layers with an average thickness of 31 nm. Thefilm with 87 nm layers had a first order band gap in the visible., Itshows a strong broad reflection in this region. The film with 31 nmlayers showed a flat reflection spectrum. The first order band gap forthis film occurs near 197 nm, deep into the UV. The narrow spikes on thereflection spectrum of the 87 nm spacing (14 mil) arise because of thedistribution of the layer thickness across the sample.

[0098] A similar SAN25/SAN20 multilayered film was carefully cleavedperpendicular to the plane of the film. The thickness of the individuallayers was then measured by atomic force microscopy (AFM). The AFM imageis shown in FIG. 6. This figure shows the layer structure clearly. Italso shows that there is a distribution of layer thickness across thesample. In this film the layer thickness distribution is qualitativelyconsistent with the width of the band gap observed in the reflectionspectrum (FIG. 5). Relatively broad band gaps are essential for opticallimiter applications. For other optical applications it will bedesirable to adjust the fabrication technique to reduce the band gapwidth.

[0099] The reflection spectrum of multilayer films where the SAN25layers were dyed with nigrosine is shown in FIG. 7. The films haveapproximately the same layer thickness as the films shown in FIG. 5. Thefilm with an 87 nm average layer thickness showed a similar band gap tothe undyed film but with a lower reflectivity in the band gap. Thenigrosine dye increases the index of the SAN25 layer so the refractiveindex contrast was smaller.

[0100] Nonlinear Optical Studies of the SAN Films

[0101] Nanosecond nonlinear transmission and Z-scan experiments wereperformed on a multilayer film consisting of alternate layers of anundyed SAN polymer and the same polymer containing 0.1% (wt/wt)nigrosine. The film had 4096 layers and the average thickness of thelayers was 93 nm. The open aperture Z-scan showed no variation intransmission with position, as expected, since this film has nononlinear absorption. FIG. 8 shows the transmission of this film for aclosed aperture (40% transmitting) Z-scan. This demonstrates a nonlinearrefraction in this sample. The shape of the scan indicates a decrease inrefractive index with fluence. This is the sign expected for the thermalnonlinearity in the dyed layers.

[0102] The relative transmission of this film as a function of incidentenergy in an f/5 optical limiter is shown in FIG. 9. In this experiment,the laser beam was brought to a focus at the center of the multilayerfilm. The transmission was approximately constant until a threshold wasreached. After the threshold, the transmission dropped rapidly to atransmission below 25%. The experiment shown in FIG. 9 was performedusing a 0.5 Hz laser repetition rate. The same transmission changes wereobserved with a 10 Hz repetition rate. This means the transmissionchanges are not due to persistent local heating of the sample. However,refractive index changes in the polymeric material with a relaxationtime of greater than 2 seconds can contribute to the observed response.

[0103] Another 1D nonlinear photonic crystal film consisted of alternatelayers of an undyed SAN25 polymer and the same polymer with 0.5% (wt/wt)lead tetrakis(cumylphenoxy)phthalocyanine (PbPc(CP)₄). It was a 4096layer film with an average layer thickness of 93 nm. The leadtetrakis(cumylphenoxy)phthalocyanine dye was substantially molecularlydispersed in the polymeric material. An open aperture Z-scan recorded asa function of increasing energy on this sample in an f/5 optical systemat 532 nm was shown in FIG. 10. This figure shows transmission as afunction of film position at increasing input fluences. This shows thatthis multilayer film exhibits a strong nonlinear absorption. This is inaddition to the nonlinear refraction that appears in the nigrosine dyedsample. At the highest energy shown, an additional drop in transmissionis seen that is consistent with a nonlinear refraction contribution.

[0104] Polycarbonate Nonlinear 1D Photonic Crystals

[0105] Several nonlinear nanolayered polymeric films were fabricatedusing a polycarbonate host. The nonlinear response was introduced intothe alternate layers by dissolving nigrosine or leadtetrakis(cumylphenoxy)phthalocyanine (PbPc(CP)₄) dye in the polymericmaterial used for those layers. In these films, the same polymericmaterial was used for the dyed and undyed layer. The nonlinear dyecauses a slight increase in the refractive index of the dyed layer, soindex of refraction of the dyed layers was slightly greater than that ofthe undyed layer. This led to a small linear reflectivity of the filmsin the band gap. This is shown in FIG. 11 which gives the reflectionspectrum of two multilayer films consisting of alternate layers of anundyed polycarbonate and the same polymeric material with 0.2% (wt/wt)nigrosine. In this figure, the reflectivity of a 14 mil film with anaverage layer thickness of 87 nm shows a broad reflectivity in the 500nm to 900 nm region. This reflectivity was absent in the 5 mil film witha layer thickness of 31 nm.

[0106] Nonlinear Optical Studies of the Polycarbonate 1D PhotonicCrystal Films

[0107] Nanosecond nonlinear transmission and Z-scan transmissionexperiments were performed on the first multilayer film shown in FIG.11. It consisted of alternate layers of an undyed polycarbonate and thesame polymeric material with 0-2% (wt/wt) nigrosine. The film had 4096layers and an average layer thickness of 87 nm. The nonlineartransmission of this film was compared to a similar film in which allthe layers were dyed. In the latter film there was no modulation in thenonlinear properties across the sample.

[0108] In order to measure the nonlinear reflectivity, the sample wasmounted on a translation stage and positioned near the focus of the f/45optical system. The film was translated through the focus and thereflectivity was measured as a function of distance from focus. This isa convenient way to measure the fluence or intensity dependence of thereflectivity since the intensity and fluence of the beam vary with thedistance from the focus. The result is shown in the lower plot in FIG.12. There is about a 10% drop in reflectivity as the sample comes to thefocus. This was expected since the dyed layers initially had a largerindex than the undyed layers and the change in index of refraction withfluence is negative. The reflectivity should decrease and thetransmission should rise as the layers came closer to being indexmatched. In this experiment, the transmission of the multilayer film didincrease with incident fluence.

[0109] Two control experiments were performed. The upper plot in FIG. 12shows the reflectivity as a function of position for the film where allthe layers of the film contained the nonlinear dye. In this uniformlydyed film, the reflection was not position or fluence dependent. In thisfilm, the index remained uniform throughout the film at all fluences.This confirms that reflectivity in the first film arises because of thealternate nonlinear layers.

[0110] In addition we studied the nonlinear reflection from a filmconsisting of alternate layers of an undyed polycarbonate and the samepolymeric material with 0.2% (wt/wt) nigrosine that had 4096 layers andaverage thickness of the layers of 124 nm. In the latter film, a bandgap near 800 nm is expected. No intensity dependent reflectivity wasobserved at 573 nm. The nonlinear reflectivity was observed only in theband gap of the photonic crystal. This shows a reversible nonlinearreflectivity within the band gap in this layered film. The nanolayeredfilm behaves as a nonlinear photonic crystal.

[0111] Optical switching and the Response Time of the ReflectivityChanges.

[0112] Optical switching was demonstrated by measuring the reflectivityof a multilayer sample, in accordance with the present invention, aftera short pulse pump (control) beam was incident on the sample. Themultilayer sample consisted of alternate layers of an undopedpolycarbonate and the same polymer with 0.2% (wt/wt) nigrosine. The filmhad 4096 layers and an average layer thickness of 87 nm. The responsetime of the reflectivity was measured by studying the reflectivity as afunction of time.

[0113] The reflectivity of a multilayer sample to a probe (signal) beamat 590 nm is shown in FIG. 14 as a function of time after a 1.2picosecond pump laser is incident on the layered sample. Thereflectivity decreases with a response time of 300 to 400 picoseconds.The decrease in reflectivity occurred because the dyed layers initiallyhad a larger index than the undyed layers and the change in index ofrefraction with fluence is negative. As the layers came closer to beingindex matched, the reflectivity decreased.

[0114] The delay in the change in reflectivity can be easily understood.In this particular nanolayered material, the dominant source of therefractive index change is due to the thermal expansion of the absorbinglayer. The dynamics of the generation of the reflectivity is thenanalogous to the formation of a thermal grating in a dynamic gratingexperiment as discussed in Eichler et al (H. J. Eichler, P. Gunther, D.W. Pohl; Laser-induced Dynamic Gratings; Springer-Verlag, Berlin, N.Y.(1986)) which is incorporated herein by reference. Absorption of thepump light causes a temperature rise in the polymer layers containingthe dye. This occurs within the 1.2 picosecond pulse width of the pumplaser. The temperature rise causes the generation of acoustic waveswithin the sample. The acoustic waves are damped via acoustic absorptionto leave a static modulation in the density across the sample. This isthe “thermal grating” in dynamic grating experiments. In the layeredsamples, with a distribution of layer thickness, there is a distributionof periods for the acoustic waves. The observed reflectivity rises asthe acoustic waves damp out to leave a static variation in therefractive index across the sample.

[0115] This demonstrates that a multilayer nonlinear photonic crystalcan work as an optical switch. The response time illustrated is on theorder of a few hundred picoseconds. Faster response times can beachieved by other nonlinear mechanisms or by using a higher intensitypump beam to initiate the optical switching. This is illustrated in FIG.15 where a response time of about 10 picoseconds is achieved by a higherenergy pump beam. A response time of 2 to 10 picoseconds is expected forother nonlinear mechanisms, e.g. an excited state nonlinearity.

[0116] Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein

1. A multilayer structure comprising, a plurality of at least twoalternating layers a and b represented by formula (AB)_(x), wherex=2^(n), and n is in the range of from 4 to 15; wherein layer a iscomprised of component (a) and layer B is comprised of component (b);wherein at least one of components (a) and (b) is an organic polymericmaterial containing a nonlinear dye and exhibiting nonlinear opticalresponse; and wherein said organic polymeric material is selected fromthe group consisting of polyethylene naphthalate, isomers ofpolyethylene naphthalate, polyalkylene terephthalates, polyetherimides,a styrenic polymer, a polycarbonate, a poly(meth)acrylate, a cellulosederivative, a polyalkylene polymer, a fluorinated polymer, a chlorinatedpolymer, a polyethersulfone, polyacrylonitrile, a polyamide,polyvinylacetate, a polyetheramide, a styrene-acrylonitrile copolymercontaining between 10 and 50 wt % acrylonitrile, a styrene-ethylenecopolymer, poly(ethylene-1,4-cyclohexylenedimethylene terephthalate) andblends thereof.
 2. The multilayer structure of claim 1, comprising aplurality of at least three alternating layers A, B and C, representedby formula (ABC)_(x), wherein layer A is comprised of component (a),layer B is comprised of component (b) and layer C is comprised ofcomponent (c); and wherein said components (a), (b) and (c) may be thesame or different, provided that at least one of the components (a), (b)and (c) exhibits nonlinear optical response:
 3. The multilayer structureof claim 1, wherein the difference in the refractive index of component(a) and component (b) is in the range of 0 to 10%.
 4. The multilayerstructure of claim 1, wherein components (a) and (b) have a transmissionwindow within the range of about 200 micrometers and about 350nanometers.
 5. The multilayer structure of claim 1, wherein at least oneof the components (a) and (b) is a polymeric composite material.
 6. Themultilayer structure of claim 1, wherein said nonlinear dye affectschange in the refractive index by thermal change.
 7. The multilayerstructure of claim 1, wherein said nonlinear dye affects change in therefractive index by optical pumping.
 8. The multilayer structure ofclaim 1, wherein said nonlinear dye is selected from the groupconsisting of a phthalocyanine, a naphthalocyanine, a porphyrin, anorganometallic cluster compound, a fullerene, leadtetrakis(cumylphenoxy)phthalocyanine, leadtetrakis(polydimethylsiloxane) phthalocyanine,tetra-tert-butyl(p-trifluoromethyl phenyl) indium(III), phthalocyanineand bis(trihexylsiloxy)silicon naphthalocyanine.
 9. The multilayerstructure of claim 1, wherein said multilayer structure contains atleast 30 layers.
 10. The multilayer structure of claim 1, wherein thethickness of each layer is in the order of from 5 nanometers and 10micrometers.
 11. The multilayer structure of claim 1, which includes atie layer (T) between layer A and layer B; said multilayer structurerepresented by formula (ATBT)_(x)A, where x=2^(n) and n is the number ofmultiplier elements.
 12. The multilayer structure of claim 1, whichincludes a barrier layer.
 13. The multilayer structure of claim 1, whichincludes a surface layer on at least one major surface thereof
 14. Themultilayer structure of claim 1, wherein said multilayer structure is afilm or a sheet.
 15. The multilayer structure of claim 1, wherein saidmultilayer structure is an optical limiter.
 16. The multilayer structureof claim 1, wherein said multilayer structure is an optical switch. 17.The multilayer structure of claim 21, wherein the response issue of saidoptical switch is at least 20 femtosecond.
 18. A method for forming themultilayer structure of claim 1, comprising extruding component (a) inan extruder (A) to form a melt stream (A) and component (b) in anextruder (B) to form a melt stream (B); combining melt stream (A) withmelt stream (B) in a feedblock to form parallel layers (A) and (B);advancing said parallel layers through a series of multiplying elements(n) to form the multilayer structure.
 19. A multilayer structurecomprising, a plurality of at least two alternating layers A and Brepresented by formula (AB)_(x), where x=2^(n), and n is in the range offrom 4 to 15; wherein layer A is comprised of component (a) and layer Bis comprised of component (b); wherein at least one of components (a)and (b) exhibits nonlinear optical response; and wherein said component(a) is an oligomeric material (A) and said component (b) is anoligomeric material (A) containing a nonlinear dye.
 20. A multilayerstructure comprising, a plurality of at least two alternating layers Aand B represented by formula (AB)_(x), where x=2^(n), and n is in therange of from 4 to 15; wherein layer A is comprised of component (a) andlayer B is comprised of component (b); wherein at least one ofcomponents (a) and (b) exhibits nonlinear optical response; and whereinthe difference in the refractive index of component (a) and component(b) is 0.